The invention relates to a process and an apparatus for recycling of PET-material.
Under PET-material (material of polyethylene terephthalate) all PET-materials and/or PET-objects are to be understood, also those of BOPET (bi-oriented PET), in particular pieces, milled bottle material, foils, fibers, flakes and the like.
When recycling articles of polyethylene terephthalate or of polyethylene terephthalate materials it is of importance that the PET-material to be recycled is as dry as possible in order to avoid a hydrolytic degradation of the molecule chains by the influence of hydrogen during plastifying or during the melting step. However, an efficient drying is possible only at elevated temperature at which the amorphous PET-particles or PET-parts adhere to each other. For these reasons, a crystallization of the PET-material should be obtained before drying. Such a crystallization can be obtained by uniformly moving or mechanically treating the particles at a temperature lower than the drying temperature, in any case at a temperature that is lower than the plastifying temperature.
However, since the PET-materials to be recycled or the articles consisting of PET, in particular PET-bottles, are mostly soiled and are washed and possibly previously comminuted, at which procedure a simultaneous soiling frequently cannot be avoided, in most cases the material is discretely comminuted or milled, washed and dried. Such a pre-drying should be at least so efficient that the moisture content does not exceed a value of 1.5 weight-% of the PET-material to be treated or to be recycled.
In particular, it is an object of the invention to provide for a process and an apparatus of the initially described kind by which PET-material is subjected to a quick recycling that is as power saving as possible, wherein the PET-granulate produced from the melt obtained, or PET-articles made therefrom, have high viscosity values, in particular a viscosity that can be compared with the viscosity values of the PET-material to be recycled. Further, the obtained melt or, respectively, the PET-granulate produced from the melt should meet the rules concerning food stuff techniques, which means that harmful substances or contaminations contained in the PET-material that is supplied to the recycling process are eliminated as far as possible by a suitable treatment.
This object is achieved within a process and the apparatus of the present invention.
Therefore, a two-step treatment of the delivered or supplied PET-material is made, wherein, when pre-treating in a pre-treatment apparatus, no plastification of the PET-material is made, however a crystallization and a certain pre-densification at a simultaneous drying takes place. The pre-densification is obtained by a suitable temperature by mechanical treatment of the PET-material or by power introduction into it. In particular, increasing or controlling of the temperature is made by the mechanical treatment of the PET-material or, respectively, by conversion of the rotational energy of at least one revolving mixing and/or comminuting element into thermic energy by reasons of the friction losses occurring.
During the main treatment in a main treatment apparatus, the PET-material is further dried and crystallized at an elevated temperature and is kept under a high vacuum for a certain mean dwell time. Again there is a mechanical treatment or material condensation and introduction of power by at least one revolving mixing and/or comminuting element that by its rotation introduces the corresponding thermic energy into the PET-material and further heats it.
The main treatment that is effected under vacuum reduces the remaining humidity to a predetermined defined mean value and also has the effect that volatile harmful substances are separated from the PET-material.
The temperature at the main treatment is kept below the melting temperature of the PET-material, in particular about 40 to 60xc2x0 C. below the melting temperature. However it is desired to keep this temperature as high as possible.
Only after the main treatment, the PET-material conveyed off is plasticized by an extruder that preferably is directly connected to the main treatment apparatus. By the direct, vacuum-tight connection, the vacuum within the main treatment apparatus can act into the inlet section of the extruder. This extruder comprises a plasticizing zone followed by a compression and damming zone. This damming zone is followed by a degassing or evacuating zone in which volatile substances are sucked off from the melt by a vacuum, in particular by a high vacuum. Within this, a one-step or multi-step degassing can be provided. It is also possible to provide a plurality of compression zones and decompression zones having different values of the vacuum one after the other. Thereby also persistent contaminations or those which are difficult to vaporize can be vaporized off.
By a suitable selection of the temperatures and of the dwell times within the pre-treatment apparatus and within the main treatment apparatus, the viscosity value of the melt obtained from the extruder and of the PET-granulate produced from the melt can be adjusted. By suitably long dwell times and suitably high temperatures within the vacuum, a positive influence onto the viscosity is effected or, respectively, a re-polymerization takes place.
Advantageous embodiments of the invention can be seen from the following description and the drawing. In the drawing, two exemplary embodiments of the invention are shown.